Atomic resolution structures of Rieske iron-sulfur protein: Role of hydrogen bonds in tuning the redox potential of iron-sulfur clusters

The Rieske [2Fe-2S] iron-sulfur protein of cytochrome bc(1) functions as the initial electron acceptor in the rate-limiting step of the catalytic reaction. Prior studies have established roles for a number of conserved residues that hydrogen bond to ligands of the [2Fe-2S] cluster. We have constructed site-specific variants at two of these residues, measured their thermodynamic and functional properties, and determined atomic resolution X-ray crystal structures for the native protein at 1.2 angstrom resolution and for five variants (Ser-154 -> Ala, Ser-154 -> Thr, Ser-154 -> Cys, Tyr-156 -> Phe, and Tyr-156 -> Trp) to resolutions between 1.5 angstrom and 1.1 angstrom. These structures and complementary biophysical data provide a molecular framework for understanding the role hydrogen bonds to the cluster play in tuning thermodynamic properties, and hence the rate of this bioenergetic reaction. These studies provide a detailed structure-function dissection of the role of hydrogen bonds in tuning the redox potentials of [2Fe-2S] clusters.